An enhanced mobility wheelchair with semi-autonomous control logic includes criteria for selecting level, stair, and slope modes. Each mode progresses through criteria-invoked phases as the wheelchair travels. Each axis performs an assigned duty for each mode and phase combination to seek a defined target. Control logic invokes stair mode from the level mode when steps are sensed. Criteria restricts negotiating obstacle angles and heights encountered beyond capability and permits reversed travel to exit obstacle. Front and rear support systems include the ability to sense both forward and backward facing steps by adding distance sensors, sensing linkage and rotary sensors. Legrest adjustment raises the feet for forward step climbing and shortens the wheelchair length for tight turns. Steep down slope and steep up slope modes are invoked from a small angle slope mode. Absolute measurement of spider rotation and spider cam engagement does not require adjustment.
